feat(http/unstable): accept HeadersInit for serveDir and serveFile headers

[minato32]

Closes #6723.

ServeDirOptions.headers and ServeFileOptions.headers in @std/http/unstable-file-server now accept HeadersInit instead of string[], so callers can pass a Headers instance, a record, or a list of tuples without hand-rolling name:value strings.

The stable serveDir/serveFile keep their existing string[] signature, so the file-server CLI behaviour is unchanged. The unstable wrapper strips headers before delegating to the stable implementation and applies them to the response itself (skipping redirect responses, matching the stable behaviour).

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

❌ Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.56%. Comparing base (cdf74a8) to head (2599d13).

Files with missing lines	Patch %	Lines
http/unstable_file_server.ts	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7190      +/-   ##
==========================================
- Coverage   94.57%   94.56%   -0.01%     
==========================================
  Files         636      637       +1     
  Lines       52142    52158      +16     
  Branches     9401     9404       +3     
==========================================
+ Hits        49315    49325      +10     
- Misses       2249     2254       +5     
- Partials      578      579       +1     

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